The present invention relates to a method and apparatus for directly or indirectly applying a liquid or pasty application medium to one or both sides of a continuous surface.
Methods according to the class and apparatus according to the class are usually used as part of paper-making machines or coating plants in order to provide a continuous surface, for Instance a material web composed e.g. of paper, cardboard or a textile material, with one or more layers of application medium, e.g. dye, starch, impregnating fluid or the like, on one or both sides. So-called direct application involves the application of liquid or pasty application medium from an applicator directly onto the surface of the continuous material web which during application is supported on a revolving support surface, e.g. a continuous belt or a counter-roll. Indirect application of the medium, on the other hand, first involves the application of liquid or pasty application medium onto an opposite surface which acts as a carrier face, e.g. the surface of a counter-roll designed as an application roll from where it is transferred in a roll gap, through which the material web passes, from the application roll onto the material web.
To perform the direct or indirect application techniques described above, use is normally made of application devices which either have an application chamber or are fitted with a single nozzle or free-jet nozzle extending in the form of a narrow, long gap across substantially the entire web width.
A method and an apparatus used to apply a single tracexe2x80x94which is as far as possible narrowxe2x80x94of liquid or pasty medium, preferably glue, onto a continuous material web are known from DE 195 04 652 A1. For this purpose, the apparatus has a nozzle head with a single nozzle from which the medium is sprayed directly and without contact onto a narrow partial region of the material web.
A method, comparable with the technical solution according to DE 195 04 652 A1, and its associated apparatus are also known from DE 295 06 334 U1. The method according to DE 295 06 334 U1 also involves the application of just one strip-like trace of application medium.
The present invention is based upon the object of providing an innovative, simple and effective method of directly or indirectly applying a liquid or pasty application medium onto one or both sides of a continuous surface essentially over the entire surface area. Another object is to make available a suitable apparatus for performing this method.
According to this method of directly or indirectly applying a liquid or pasty application medium onto one or both sides of a continuous surface, the application medium is applied onto the surface in a plurality of single application regions by means of a plurality of single application nozzles spaced apart from one another side by side and/or in succession in the direction of width and/or longitudinal direction of the surface and clearly distanced from the surface; the application medium emerges from each of these nozzles, whereby adjacent single application regions respectively intersect in their respective edge regions at least to an extent, thus producing a layer of application medium across essentially the entire width of the surface to be coated.
In accordance with the invention, the continuous surface may be a material web, particularly one made of paper or cardboard, (e.g. in the case of direction application) or it may also be the surface of an application roll (e.g. in the indirect technique) or another revolving support or carrier face. The application medium is applied without contact from the respective single application nozzles onto the continuous surface, i.e. there is no direct contact between the nozzles and the surface. In the method according to the invention, the single application nozzles form a nozzle array which extends substantially in the direction of width of the continuous surface or at an angle thereto. The nozzles can be evenly or unevenly arranged in a row and can also be staggered in relation to the continuous surface""s longitudinal direction. In this way, arrays with curved sections, such as a wavelike array etc., are possible in addition to a straight array. In a specific configurational pattern, the single application nozzles can also be distributed across substantially the entire width of the continuous surface to be coated, whereby apparent overlaps of individual sections of the course of the array are also possible. As compared with the known prior art, the entire application covering substantially the entire surface area is therefore made upxe2x80x94in the method according to the inventionxe2x80x94of a plurality of small single application regions which are primarily obtained from the ejection geometry and ejection characteristic of the respective single application nozzles. A number of nozzle types is in principle conceivable as suitable application nozzles. The following are feasible examples: nozzles which generate a free jet, i.e. a xe2x80x9ccontinuous curtainxe2x80x9dxe2x80x94running through the ambient atmospherexe2x80x94of the ejected application medium, spray nozzles which atomize the application medium, including sprayers with electrostatic and/or mechanical atomizers, e.g. high-rotation bell-type spray systems, and the like.
The respective edge regions of the coated surface""s adjacent single application regions can be intersected either in that the application medium ejected by two adjacent single application nozzles respectively in a single operating cycle is intersected before or during application onto the surface, i.e. by overlapping the spray cones or jets of these nozzles, or in that single application regions only overlap in consecutive operating cycles, e.g. by two successive and xe2x80x9cstaggeredxe2x80x9d nozzle arrays.
The intersection or overlapping of the respective edge regions of the single application regions produced by the single application nozzles is preferably adapted to one another with such precision that an even layer thickness is obtained over substantially the entire width of the coated continuous surface. This is not, however, absolutely necessary. Particularly in the case of indirect application, it is possible for the application produced by the single application nozzles initially to be still somewhat striped and for it not to be evened out until afterward in the subsequent roll gap or by using metering and/or evening-out means downstream of the nozzles.
The method according to the invention can be performed using particularly simple and inexpensive structural design means, making it possible to produce an even and top-quality application over the entire surface area in a simple and effective manner both in a direct and in an indirect application process,. Since the entire application is, as explained above, composed of a number of small single application regions formed by the single application nozzles, not only a longitudinal profile but also a transverse profile of the application that is generated or is to be generated canxe2x80x94if necessaryxe2x80x94in a way be set, manipulated or at least pre-regulated to a considerable extent in a common procedural step. Since the single application nozzles can also in principle be individually controlled, an effective control and/or regulating concept can be implemented in a particularly simple way in the method according to the invention.
The application medium is preferably applied to the surface essentially without excess; for this purpose, only as much application medium is ejected from the single application nozzles as is needed to build up the predetermined layer thickness.
As concerns specific applications, however, it has also proved advantageous to apply an excess of application medium according to another possible embodiment version of the invention, with the amount of application medium preferably corresponding to 2 to 5 times the final application to be achieved. The invention is not, however, exclusively restricted to the aforementioned quantitative data. If necessary, it is by all means possible to exceed or fall short of these values. In each case, it is also optionally provided that the application medium applied in excess should be doctored by at least one doctor element and returned to an application-medium loop. As already briefly indicated earlier on, it is also provided, as Dart of the above invention according to at least another embodiment, that the application medium applied to the continuous surface should be evened out by at least one evening-out means. In practice, doctor elements such as doctor blades, doctor bars, roller doctor elements or the like are used as evening-out means and/or to doctor any excess application medium. If a doctor bar, particularly a smooth doctor bar, is used, it is expedient for this bar to have as large a diameter as possible. For the purpose of effectively doctoring, evening out, and setting a specific coating weight or a specific coating thickness, such a doctor element does in fact require a correspondingly high dynamic pressure to float on the surface. This pressure has to be generated by the kinetic energy and impulse of the liquid or pasty application medium coating film previously applied by the single application nozzles to an application roll (in the case of indirect application). The coating film""s low mass in the case of application by means of the single application nozzles makes it necessary for the area below the doctor bar to be large, particularly so as to ensure that the doctor bar floats and hence to ensure systematic doctoring even in the case of low machine speeds or low advancing speeds of the continuous surface and in the case of a low film impulse. Instead of the smooth doctor bar, doctor bars with a grooved or rough surface can also be used in principle. The smooth doctor bar is nevertheless recommended for improved floating characteristics. If possible, the doctor bar""s diameter should be at least 14 mm, though preferably approx. 35 mm. The invention is not, however, restricted to these dimensions. Depending on the particular application, it is by all means possible to exceed or fall short of the above values.
The aforementioned object is also solved by an apparatus according to the invention comprising the features described as follows. This apparatus for directly or indirectly applying a liquid or pasty application medium onto one or both sides of a continuous surface comprises a plurality of single application nozzles spaced apart from one another side by side and/or in succession relative to the direction of width and/or longitudinal direction of the surface; the application medium emerges from each of these nozzles which are clearly distanced from the surface. The apparatus according to the invention also provides those advantages already discussed in conjunction with the method according to the invention. The apparatus according to the invention can also be implemented in a particularly simple and inexpensive manner in terms of structural design; it has a very sturdy structure and can be handled and serviced more easily due to its simpler structure.